To produce castings by gravity pouring, a casting mold composed of sand particles, which is a gas-permeable casting mold (a so-called sand mold), is most generally used. With such a gas-permeable casting mold, a gas (generally air) remaining in a cavity of a particular shape is pushed out of the cavity by a metal melt (simply called “melt”), and the melt is formed into a casting having substantially the same shape as the cavity. The cavity of the casting mold generally includes a sprue, a runner, a feeder and a product-forming cavity, into which a melt is supplied in this order. When a melt head in the sprue becomes high enough to fill a product-forming cavity, the pouring of the melt is finished.
A solidified melt forms a casting integrally extending from the sprue to the runner, the feeder and the product-forming cavity. The feeder is not an unnecessary portion for obtaining good castings, while the sprue and the runner are merely paths for a melt to reach the product-forming cavity, which need not be filled with the melt. Thus, as long as a melt is solidified in a state of filling the sprue and the runner, drastic improvement in a pouring yield cannot be expected. In the case of castings integrally having unnecessary portions, considerable numbers of steps are needed to separate cast products from unnecessary portions, resulting in low production efficiency. Accordingly, the sprue and the runner pose large problems in increasing efficiency in gravity casting.
A revolutionary method for solving the above problems is proposed by JP 2007-75862 A and JP 2010-269345 A. To fill a desired cavity portion, part of a cavity in a gas-permeable casting mold, this method pours a metal melt in a volume smaller than that of an entire cavity in a gas-permeable casting mold (hereinafter referred to as “casting mold cavity”and substantially equal to that of the desired cavity portion, into the cavity by gravity; supplies a gas (compressed gas) into the cavity through a sprue before the melt fills the desired cavity portion; and then solidifies the melt filling the desired cavity portion. By this method commonly disclosed in JP 2007-75862 A and JP 2010-269345 A, which may be called “pressure-casting method,” it is expected to make it substantially unnecessary to fill a sprue and a runner with a melt, because pressure to be obtained by the melt head height is given by the compressed gas.
As a result of experiment to follow the pressure-casting method described in JP 2007-75862 A and JP 2010-269345 A, the inventors have found that because a melt filling the cavity under gas pressure flows reversely when the supply of the gas is stopped, the supply of the gas should be continued until part of the melt in contact with the gas is solidified, though the entire melt need not be solidified, to obtain sound castings. However, it takes much time until a portion of the melt in contact with the gas is solidified to stop the reverse flow of the melt. Accordingly, an additional means for preventing the reverse flow of the melt to keep the melt to fill the cavity is necessary to shorten a production tact in such method.
As such additional means, JP 2007-75862 A and JP 2010-269345 A disclose various methods, such as a method of supplying a cooling gas to a portion of the melt in contact with the gas to accelerate the solidification of the melt, a method of mechanically shutting the cavity, a method of filling refractory particles, and a method of introducing a metal to accelerate the solidification of the melt by the latent heat of melting the metal. Though any of them are effective, for example, the method of supplying a cooling gas may suffer a problem that the melt is not solidified within a desired time, because of insufficient heat capacity of the cooling gas depending on the size of a casting. A method of mechanically sealing a melt by a shutter plate projecting into a runner from a recess open on an upper mold surface above the runner is also disclosed. In this method, however, the shutter plate should be provided in every casting mold, suffering cost increase. Thus desired is a simpler means capable of exhibiting sufficient effects.